FIG. 9(a) is a cross-sectional view of a ridge type semiconductor laser disclosed by J. Hashimoto et.al. in IEEE Journal of Quantum Electron, Vol.33, pp.66-77, 1997. This laser comprises a p side electrode 1, an insulating film 2, a p type GaAs contact layer 3, a p type GaInP upper cladding layer 4 having a stripe-shaped ridge extending in the resonator length direction, a ridge side undoped GaInAsP second guide layer 5, a ridge side undoped GaAs first guide layer 6, an active layer 7, a substrate side undoped GaAs first guide layer 8 having the same composition ratio and thickness as those of the ridge side first guide layer 6, a substrate side undoped GaInAsP second guide layer 9 having the same composition ratio and thickness as those of the ridge side second guide layer 5, an n type GaInP lower cladding layer 10 having the same composition ratio as that of the upper cladding layer 4 and the same thickness as that of the ridge of the cladding layer 4, an n type GaAs buffer layer 11, an n type GaAs substrate 12, and an n side electrode 13. FIG. 9(b) is a graph showing the refractive index profile of the semiconductor laser in the direction perpendicular to the surface of the substrate 12. In FIG. 9(a), "z" shows the resonator length direction, "x" shows the direction perpendicular to the surface of the substrate 12 (hereinafter referred to as "thickness direction"), and "y" shows the direction perpendicular to both of the resonator length direction z and the thickness direction x (hereinafter referred to as "width direction").
A description is given of the operation of the semiconductor laser. Holes and electrons are injected through the upper cladding layer 4 and the lower cladding layer 10 into the active layer 7, respectively, and recombine to generate light. The light so generated is propagated along the resonator length direction z while being influenced by the refractive indices in the thickness direction x and the width direction y, and it is amplified while being reflected between the facets of the laser, resulting in laser oscillation.
In this prior art semiconductor laser, the refractive index distribution in the thickness direction x is symmetrical about the active layer 7, until reaching the upper cladding layer 4 and the lower cladding layer 10 which are disposed on and beneath the active layer 7, respectively. That is, as shown in FIG. 9(b), the ridge side first guide layer 6, the ridge side second guide layer 5, and the upper cladding layer 4 have the same refractive indices and the same thicknesses as those of the substrate side first guide layer 8, the substrate side second guide layer 9, and the lower cladding layer 10, respectively.
As described above, in the prior art ridge type semiconductor laser, since the refractive index distribution in the thickness direction x is symmetrical about the active layer 7, the propagated light is distributed almost symmetrically about the active layer 7 in the thickness direction x. However, a refractive index present is generated in the width direction y because of the ridge of the upper cladding layer 4, when the propagated light is distributed almost symmetrically about the active layer 7 as described above, the influence of the refractive index in the width direction y on the propagated light at the ridge becomes significant, whereby a higher mode of oscillation is permitted. As a result, kinks occur due to mode competition during low-power output operation, and output power cannot be increased in the practical use.